1. Field of the Invention
The present invention relates to apparatuses and methods for the detection and measurement of electrical discharge.
2. Description of the Related Art
Unwanted electrical discharges through a nominally insulating medium are a commonly-occurring fault in high-voltage electrical systems. Examples of such discharges are electrical arcs in air, sulfur hexafluoride, or insulating oil. xe2x80x9cPartial dischargexe2x80x9d can also occur. Partial discharge refers to electrical charge flow that does not completely bridge the gap between two conducting electrodes. The current flow in a partial discharge is typically many orders of magnitude lower than the current flow in an electrical arc. Partial discharges include both xe2x80x9csurfacexe2x80x9d partial discharges (i.e. corona) on the surface of conductors or insulators in air or other gaseous dielectrics, and xe2x80x9cinternalxe2x80x9d partial discharges in confined spaces or voids, beneath the surface of a device, e.g. in insulating oil or gas-filled voids in solid insulation.
xe2x80x9cCoronaxe2x80x9d or xe2x80x9ccorona dischargexe2x80x9d denotes a type of partial discharge occurring in large, unconfined spaces such as exterior equipment surfaces. A corona discharge often occurs around high-voltage electrical apparatus when the electric field in the surrounding air exceeds the threshold for dielectric breakdown of air. When this occurs, the air surrounding the high-voltage apparatus ceases to be an insulator and becomes partially conducting. For example, apparatus such as electrical power transmission lines, transformer and substation insulators and bushings, high-voltage power supplies, and the like often have coronas associated therewith.
Although such electrical discharges are sometimes a normal and harmless feature of equipment operation, they are often a symptom or a cause (or both) of insulation system failure. In particular, a relatively small amount of electrical charge flow in such discharges may be normal and relatively harmless, whereas a comparatively large amount may be abnormal and indicative of a fault condition. Quantitative measurement of electrical discharge magnitudes can thus be used to detect faults, to aid in operating and maintaining equipment such as high-voltage electrical systems.
For example, an excessive partial-discharge magnitude can be a warning of contamination of transmission or distribution insulators and bushings, which can lead to destructive xe2x80x9cflashoverxe2x80x9d of the insulator, or to erosion and failure in the case of polymer insulators. Excessive partial discharge may also be a symptom of failure of one segment of a xe2x80x9cstringxe2x80x9d of ceramic suspension insulators, which places additional stress on the other segments and may evolve into total failure of the entire xe2x80x9cstringxe2x80x9d if left uncorrected. Excessive partial discharge on the stator windings of generators and motors can also lead to destructive failure of the winding insulation.
Electronic techniques are commonly employed to quantitatively measure electrical discharge, for example in high-voltage electrical equipment. Such electronic techniques are often used to measure, for example, corona and other partial electrical discharges. Conventional electrical discharge measurement instruments measure the charge or current flow within selected time and/or frequency windows, at the point of electrical connection between a discharge-measurement instrument and the device under test (DUT). Such techniques may be used for quality assurance testing in laboratories or factories, for example, and can measure both discharges occurring within insulation (partial discharge) and discharges occurring on the outside surface of conductors or insulators (corona, a specific type of partial discharge). The most common such electronic technique is termed partial-discharge testing, and is described in detail in IEC Standard 270 (Partial Discharge Measurements) and in IEEE Draft Standard P1434 (Measurement of Partial Discharges in Rotating Machinery).
Unfortunately, these conventional electronic discharge measurement techniques and devices suffer from certain disadvantages. First, their measurement sensitivity depends upon the location of the discharge and the capacitance of the equipment, and thus may be very low for equipment of large size. Second, since physical connection is required between the device under test and the measuring instrument, the equipment must be disconnected from service in order to allow these connections to be made. This can be inconvenient or impossible if the equipment is part of an operating power grid, and may also be time-consuming and expensive. Third, conventional electronic discharge measurement techniques have difficulty in discriminating between actual partial discharges and ambient electronic noise, particularly when measurements are made outside of a test laboratory or on operating equipment; false or ambiguous readings are therefore not uncommon. Fourth, the electronic method generally cannot provide any information about the location of the discharge (as opposed to its magnitude); this location information would greatly facilitate analysis and repair of the defects associated with excessive discharge. Improved discharge-measurement devices and methods which do not suffer the above-mentioned drawbacks are therefore needed.
An electrical discharge measurement device for non-contact measurement of an electrical discharge having a discharge magnitude. The electrical discharge causes a corresponding emission of optical radiation. The electrical discharge measurement device has a light-collecting device for forming an image from the optical radiation from the electrical discharge, and an optical filter operatively coupled to the light-collecting device for preferentially selecting wavelengths at which the electrical discharge emits relatively intense optical radiation. An optical detector receives light from the light-collecting device and filtered by the optical filter for converting the filtered light to an electrical signal. An electronic signal-processing device provides, based on the electrical signal, a quantitative discharge measurement related to the magnitude of the electrical discharge.